On Sat 2006-01-07T13:06:13 -0500, John Cowan hath writ:
> Well, yes.  But that's a matter of verbal labels.  The Gregorian calendar
> extends to all future time: what is not known is the date on which it
> will be replaced in civil use by a further refinement.  We know we will
> need one eventually, both because of the current annual discrepancy of
> about 27 seconds between the Gregorian and tropical years, and because
> of future changes in the length of the tropical year.

The changes in the length of any kind of year are slight by comparison
to the changes in length of day.  Neglecting "short" period variations
the length of the sidereal year has not changed much in a billion years.

The best plot I've found for this is on this page
http://inms-ienm.nrc-cnrc.gc.ca/en/faq_time_e.php
It is not clear that this calculation reflects the intent of any
particular agent in the schemes of calendrical design, or which.

It is relevant to note that the original design of the Gregorian
calendar did not intend to match the length of the tropical year, but
rather to keep the March equinox happening on the 21st.  As pointed
out by Duncan Steel in his book on the calendar, the length of the
tropical year (in phase with the precession) differs from the length
of the "vernal equinox year".  Their lengths differ because the
perihelion drifts with respect to the equinox.  The Gregorian
calendar was designed to match the "vernal equinox year".

All of these quantities require some agreement on the meaning of
"mean" and "short" period variations.

One aspect of Duncan Steel's book is that it relates the technical
aspects with the social aspects, and the religious ones.  Steel points
out that for cultural purposes of calendrical design, the world always
comes to an end within roughly 1000 years.

The new fields being added to GPS signals make them able to count leap
seconds for 30000 years.  That's quite an example of engineering margin.

--
Steve Allen                 <[EMAIL PROTECTED]>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99858
University of California    Voice: +1 831 459 3046           Lng -122.06014
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m

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